MAX323/MAX324/MAX325
Precision, Single-Supply,
SPST Analog Switches
6 _______________________________________________________________________________________
__________Applications Information
Logic Levels
The MAX323/MAX324/MAX325 are TTL compatible
when powered from a single +5V supply. When pow-
ered from other supply voltages, TTL compatibility is
not guaranteed, and the logic inputs should be driven
from rail to rail. For example, with a 15V supply, IN1
and IN2 should be driven low to 0V and high to 15V.
Similarly, with a 3.3V supply, IN1 and IN2 should be dri-
ven low to 0V and high to 3.3V
Driving IN1 and IN2 rail-to-rail minimizes power con-
sumption.
Analog Signal Levels
Analog signals that range over the entire supply voltage
(V+ to GND) can be switched with very little change in
on-resistance over the entire voltage range (see
Typical
Operating Characteristics
). All the switches are bidirec-
tional, so NO_, NC_, and COM_ pins can be used as
either inputs or outputs.
Power-Supply Sequencing
and Overvoltage Protection
Do not exceed the absolute maximum ratings, because
stresses beyond the listed ratings may cause permanent
damage to the devices.
Proper power-supply sequencing is recommended for
all CMOS devices. Always apply V+ before applying
analog signals or logic inputs, especially if the analog
or logic signals are not current-limited. If this sequenc-
ing is not possible, and if the analog or logic inputs are
not current-limited to <30mA, add a small-signal diode
(D1) as shown in Figure 1. If the analog signal can dip
below GND, add D2. Adding protection diodes reduces
the analog signal range to a diode-drop (about 0.7V)
below V+ (for D1), and to a diode-drop above ground
(for D2). Leakage is unaffected by adding the diodes.
On-resistance increases by a small amount at low sup-
ply voltages. Maximum supply voltage (V+) must not
exceed 17V.
Adding protection diodes causes the logic thresholds
to be shifted relative to the power-supply rails. This can
be significant when low supply voltages (5V or less) are
used. With a 5V supply, TTL compatibility is not guaran-
teed when protection diodes are added. Driving IN1
and IN2 all the way to the supply rails (i.e. to a diode-
drop higher than the V+ pin, or to a diode-drop lower
than the GND pin) is always acceptable.
Protection diodes D1 and D2 also protect against
some overvoltage situations. With Figure 1’s circuit, if
the supply voltage is below the absolute maximum rat-
ing, and if a fault voltage up to the absolute maximum
rating is applied to an analog signal pin, no damage
will result. For example, with a +5V supply, analog sig-
nals up to ±8V will not damage the circuit of Figure 1.
If only a single fault signal is present, the fault voltage
can rise to +17V or to -12V without damage occurring.
_____________________Pin Description
